U.S. Pat. No. 4,067,610, owned by the assignee of this invention, shows a chair control mechanism employing a torsion bar having a body portion extending longitudinally of the chair seat and end portions which extend tranversely of the body portion to yieldably resist pivotal movement of two components relative to each other. A torsion bar adjustment member is engageable with one of the end portions of the torsion bar and is operable to preload the torsion bar and thereby adjust the magnitude of the resistance to pivotal movement. The present invention constitutes an improvement on the chair control mechanism shown in the aforementioned patent.
In swivel-type chairs in which the chair back is pivotable relative to the seat, control mechanisms, such as the one shown in U.S. Pat. No. 4,067,610, are utilized to control the force required to tilt the chair back relative to the seat. Torsion bars are commonly utilized in the chair controls to yieldably resist such tilting movement and thus control the force requirements. Such chair control mechanisms have been used satisfactorily in the past. However, it has been discovered that as a result of "creep", which inevitably occurs in the torsion bars, further seat adjustment capability is desirable. "Creep" is a phenomenon in which permanent physical deformations of the metal occur due to prolonged exposure to stress. Such deformations can cause a decrease in the load-supporting capabilities of the chair and undesirable relaxation of the seat back.
Variations in user comfort demands have dictated that the resting (no-load) position of the seat back be adjustable to various positions. For example, a person who is typing will normally lean forward in the chair and prefer that the seat back provide firm support. A person normally leaning back in the chair may require less initial support. It is an object of the present invention, therefore, to provide a seat back tilt-adjusting mechanism to be used in cooperation with conventional chair control mechanisms to overcome the problems of creep and increase user comfort.